This application claims priority from EP 01303036.6 filed Mar. 30, 2001, herein incorporated by reference.
The invention relates generally to lithographic apparatus and more particularly to methods of providing compensation to correct lithographic errors.
In general, a lithographic projection apparatus comprises: a radiation system to supply a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure or means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-adressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from United States Patents U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in United States Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at one time; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT International Application No. WO 98/40791, incorporated herein by reference.
When performing imaging in a lithographic projection apparatus, despite the great care with which the projection system is designed and the very high accuracy with which the system is manufactured and controlled during operation, the image can still be subject to aberrations such as, for example, distortion (i.e. a non-uniform image displacement in the target portion at the image plane: the XY-plane), lateral image shift (i.e. a uniform image displacement in the target portion at the image plane), image rotation, asymmetric magnification, and focal plane deformation (i.e. a non-uniform image displacement in the Z-direction, for instance due to field curvature). Notice that, in general, image errors are not necessarily uniform, and can vary as a function of position in the image field. Distortion and focal plane deformation can lead to overlay and focus errors, for example overlay errors between different mask structures, and line-width errors. As the size of features to be imaged decreases, these errors can become intolerable.
Consequently, it is desirable to provide compensation (such as adjustment of the projection system and/or substrate) to correct for, or at least attempt to minimize, these errors. This presents the problems of first measuring the errors and then calculating appropriate compensation. Previously, alignment systems were used to measure the displacements in the image field of alignment marks. However, alignment marks typically consist of relatively large features (of the order of a few microns), causing them to be very sensitive to aberrations of the projection system. The alignment marks are unrepresentative of the actual features being imaged, and because the imaging errors depend inter alia on feature size, the displacements measured and compensations calculated did not necessarily optimize the image for the desired features.
Another problem occurs when, for instance because of residual manufacturing errors, the projection system features an asymmetric variation of aberration over the field. These variations may be such that at the edge of the field the aberration becomes intolerable.
A further problem occurs when using phase-shift masks (PSM""s). Conventionally, the phase shift in such masks has to be precisely 180 degrees. The control of the phase is critical; deviation from 180 degrees is detrimental. PSM""s, which are expensive to make, must be carefully inspected, and any masks with substantial deviation in phase shift from 180 degrees will generally be rejected. This leads to increased mask prices.
A further problem occurs with the increasing requirements imposed on the control of critical dimension (xe2x80x9cCDxe2x80x9d). The critical dimension is the smallest width of a line or the smallest space between two lines permitted in the fabrication of a device. In particular the control of the uniformity of CD, the so-called xe2x80x9cCD uniformityxe2x80x9d, is of importance. In lithography, efforts to achieve better line width control and CD uniformity have recently led to the definition and study of particular error types occurring in features, as obtained upon exposure and processing (see description above). For instance, such image error types are an asymmetric distribution of CD over a target portion, an asymmetry of CD with respect to defocus (which results in a tilt of Bossung curves), asymmetries of CD within a feature comprising a plurality of bars (commonly referred to as Left-Right asymmetry), asymmetries of CD within a feature comprising either two or five bars (commonly known as L1-L2 and L1-L5, respectively), differences of CD between patterns that are substantially directed along two mutually orthogonal directions (for instance the so-called xe2x80x9cH-Vxe2x80x9d lithographic error), and for instance a variation of CD within a feature, along a bar, commonly known as xe2x80x9cC-Dxe2x80x9d. Just as the aberrations mentioned above, these errors are generally non-uniform over the field. For simplicity we will hereafter refer to any of these error typesxe2x80x94including the errors such as, for example, distortion, lateral image shift, image rotation, asymmetric magnification, and focal plane deformationxe2x80x94as xe2x80x9clithographic errorsxe2x80x9d, i.e. feature-deficiencies of relevance for the lithographer.
Lithographic errors are caused by specific properties of the lithographic projection apparatus. For instance, the aberration of the projection system, or imperfections of the patterning structure and imperfections of patterns generated by the patterning structure, or imperfections of the projection beam may cause lithographic errors. However, also nominal properties (i.e. properties as designed) of the lithographic projection apparatus may cause unwanted lithographic errors. For instance, residual lens aberrations which are part of the nominal design may cause lithographic errors. For reference hereafter, we will refer to any such properties that may cause lithographic errors as xe2x80x9cproperties.xe2x80x9d
As mentioned above, the image of a pattern can be subject to aberrations of the projection system. A resulting variation of CD (for example, within a target portion) can be measured and subsequently be mapped to an effective aberration condition of the projection system which could produce said measured CD variation. A compensation can then be provided to the lithographic projection system such as to improve CD uniformity. A CD-control method such as described here comprises imaging a plurality of test patterns at each field point of a plurality of field points, a subsequent processing of the exposed substrate, and a subsequent CD measurement for each of the imaged and processed test patterns. Consequently, the method is time consuming and not suitable for in-situ CD control. With increasing demands on throughput (i.e. the number of substrates that can be processed in a unit of time) as well as CD uniformity, the control, compensation and balancing of lithographic errors must be improved, and hence, there is the problem of furthering appropriate control of properties.
At least one embodiment of the present invention includes a device manufacturing method comprising: providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using patterning structure to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using a projection system; obtaining information on properties of at least one of the substrate, the layer of radiation sensitive material, the projection beam, the patterning structure and the projection system; obtaining a plurality of coefficients which quantify the relationship between said properties and at least one of a plurality of lithographic errors causing anomaly in a projected image in the radiation sensitive layer; defining a merit function which weighs and sums lithographic errors; calculating a compensation to apply to at least one of the substrate, the projection beam, the patterning structure and the projection system to optimize the merit function; and applying the calculated compensation.
According to at least one embodiment of the invention, there is provided a lithographic projection apparatus comprising: a radiation system for providing a projection beam of radiation; a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; a projection system for projecting the patterned beam onto a target portion of the substrate; compensation means applicable to at least one of a holder for holding the patterning structure, the substrate table, the radiation system, the patterning structure and the projection system, to optimize a merit function which weighs and sums lithographic errors causing anomaly in a projected image in the radiation sensitive layer; and a processor for calculating at least one compensation to be applied by said compensation means on the basis of a plurality of coefficients which quantify the relationship between at least one lithographic error and properties of at least one of the patterning structure, the projection system, the radiation sensitive layer on the substrate and the projection beam.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).